Technical Field
This invention relates generally to airfoils and, more particularly, to suction boundary layer control for airfoils having surfaces with fluid passages open to a working fluid.
Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Airfoils have surfaces that react with or work on a working fluid that flows over the surfaces. For example, an airplane includes wings over which a working fluid flows to lift or sustain the airplane. In another example, a wind turbine includes a hub coupled to an electrical generator, and several blades extending radially outwardly from the hub to react with wind to turn the hub and generator and produce electricity. Similar examples exist for marine propellers, turbine engine rotors, helicopter rotors, and the like.
During relative motion between a working fluid and a surface of an airfoil, different layers of fluid flow can be identified. Of those, the fluid flow layer closest to the airfoil surface is known as the boundary layer. The boundary layer is characterized by fluid laminae that decrease in fluid velocity relative to the airfoil surface as a function of proximity to the surface. The flow in the boundary layer may be laminar wherein fluid laminae of different velocities create a smoothly-varying velocity profile to follow the contour of the surface. However, the fluid flow in the boundary layer over certain portions of the airfoil surface may become turbulent wherein additional time dependent velocity perturbations occur in the fluid while the mean flow may continue to follow the contour of the airfoil surface. A turbulent boundary layer has higher mean velocities near the surface and is less susceptible to separation than a laminar boundary layer. Where an airfoil surface turns abruptly, or a shock wave occurs, fluid flow may separate from the airfoil surface, resulting in relatively low pressure and reversed flow near the body surface, thereby contributing to increased drag, a reduction in lift, or reduced control effectiveness.
Accordingly, suction devices are used to remove boundary layer at strategic locations on airfoil surfaces to maintain attached fluid flow and thereby delay and/or reduce flow separation to reduce drag, increase lift, and/or increase control effectiveness. Conventional suction devices are effective, but are often complex, large, and/or heavy.